Endothelin receptor subtypes and their functional relevance in human small coronary arteries

1. The potent constrictor peptide endothelin (ET) has been implicated in various cardiovascular disorders including myocardial infarction and atherosclerosis. We have investigated the nature of ET receptor subtypes present on human small coronary arteries.
2. Small coronary arteries were mounted in a wire-myograph for in vitro pharmacology. To investigate the ET receptor subtypes present in different segments of the coronary vascular tree, arteries were grouped according to internal diameter. Responses in arteries with small internal diameters (mean 316.7±7.9 μm; Group B) were compared to those in larger arteries (mean 586.2±23.1 μm; Group A).
3. ET-1 consistently and potently contracted arteries from Group A and B, with EC₅₀ values of 1.7 (0.9–3.2) nM (n=15) and 2.3 (1.4–4.2) nM (n=14), respectively. No correlation was observed between ET-1 potency and internal diameter. The response to ET-1 was potently antagonized by the selective ET_A receptor antagonist PD156707 in both Group A and Group B, yielding pA₂ values of 8.60±0.12 (n=4–6) and 8.38±0.17 (n=4–6), respectively. Slopes from Schild regression were not significantly different from unity.
4. In contrast to ET-1, individual responses to ET-3 were variable. While all arteries from Group A responded to ET-3 (EC₅₀∼69 (23–210) nM) (n=12), no response was obtained in 5 of the 14 tested in Group B. Of those responding, many failed to reach a maximum at concentrations up to 1 μM. ET-1 was more potent than ET-3 in all arteries tested. A biphasic ET-3 response was observed in 8 arteries suggesting that a small ET_B population was also present in some patients. The selective ET_B receptor agonist sarafotoxin S6c had little or no effect up to 10 nM (n=4–6).
5. Responses to ET-1 and ET-3 were unaffected by removal of the endothelium in arteries from both groups suggesting a lack of functional, relaxant ET_B receptors on endothelial cells (n=5).
6. Using autoradiography, specific high density binding of the non-selective, ET_A/ET_B ligand [¹²⁵I]-ET-1 and selective ET_A ligand [¹²⁵I]-PD151242 was detected on the vascular smooth muscle layer of small intramyocardial coronary arteries (n=5). In contrast, little or no binding of the selective ET_B receptor ligand [¹²⁵I]-BQ3020 was observed (n=5). Similarly, [¹²⁵I]-ET-1 binding to vascular smooth muscle was absent in the presence of the selective ET_A receptor antagonist PD156707.
7. We conclude that human small epi- and intramyocardial coronary arteries express predominantly ET_A receptors and it is these receptors which mediate ET-induced contractions. A constrictor ET_B receptor population may exist in some patients. However, these receptors may have a limited role as contractions to ET-1 can be blocked fully by the selective ET_A receptor antagonist PD156707.